Within major production facilities, highly viscous fluids, such as for example polymer melts, polymer solutions or food preparations, are generally transported in closed pipeline systems. With the delivery pressures usually occurring, these fluids are to be regarded as incompressible. At the same time, the pipeline systems are themselves constructed with a constant volume, to be able to withstand the delivery pressure and to allow exact control of the volume flow delivered.
Therefore, the throughput of the highly viscous fluid only changes along a closed pipeline system when additional materials are fed into the pipeline system or material is discharged from the pipeline system.
The former can be achieved by units which build up pressure, such as for example extruders or pumps, the latter by valves, drain cocks or processing tools, such as for example nozzles or injection molds. The type of units and valves used and how they are interconnected to form process engineering installations is very varied.
Therefore, such an installation is described below by taking the example of processing hot-melt adhesives.
At the beginning of the process, the adhesive composition to be processed is produced from raw materials and intermediate products in accordance with its formulation. Used for example for this are mixing tanks with special mixing units or kneading machines, compounding extruders and numerous further types of machine. If a ready-made adhesive composition is used, it must be melted in a conserving and uniform manner. This takes place for example in so-called tank melting devices, in drum melting installations, in roller melting devices or in melt extruders. From these devices, the finished hot-melt adhesive is fed into a pipeline or a pressure-resistant hoseline by a pump or under the unit's own pressure. Permanently installed in this pipeline are usually a number of shut-off valves, measuring points for pressure and temperature, melt filters and much more besides. If changing hot-melt adhesives or very large amounts are being processed, for the adhesive compositions there are often a number of production lines, which are connected to one another by means of corresponding pipe switches and shut-off valves.
The melt passes through this pipeline system directly into one or more application systems. Part of these application systems is in most cases a dedicated pressure-building element, such as for example a gear pump, a piston metering device or a conveying screw. In some cases, these pressure-building elements are dependent on a constant minimal admission pressure for them to be operational. This is often the case for example with gear pumps. Systems for applying the molten adhesive composition to a carrier material may be, for example, continuously operating nozzles, for instance for full-area coating or for strip coating, for bead application or for vortex spraying. Equally, intermittent application methods are used, in which the adhesive stream is interrupted in predetermined cycles or if need be manually, such as for example in the fully automatic gluing of cardboard packs or in manual application by means of an adhesive gun. Only in very rare cases can it be assumed that the production and processing of the hot-melt adhesives takes place with exactly the same throughputs. Even in these ideal cases, it must be taken into account that even small differences in throughput or pulsations cause considerable pressure fluctuations in the completely filled pipeline systems if elements with inflexible delivery, such as for example melt pumps or extruders, are used as the delivery means.
Fluctuating admission pressures, however, considerably impair for example the constancy of the application of the composition in the case of full-area nozzle coating. Therefore, sophisticated control loops are required to protect the installation and to ensure a high product quality and a stable process.
Much more frequently, discontinuous production of hot-melt adhesives is combined with continuous processing. Encountered more rarely are combinations of continuous production and discontinuous processing. In both cases, an equalization of the different throughputs must be provided by the machinery or equipment used.
Furthermore, when the production and processing of hot-melt adhesives are combined, short-term failure of one of the parts of the installation also leads to the immediate failure of the other, still operational part of the installation, due to the completely filled pipeline system, since either the adhesive composition cannot be accepted or no adhesive composition is available.
In all the cases outlined here, the situation can be remedied by the use of units which are not inflexible in their delivery, or by correspondingly designed equalizing volumes. Such equalizing volumes may be buffer tanks or pulsation dampers.
The use of units which are not inflexible in their delivery, such as for example pneumatic piston pumps or gear pumps with low volumetric efficiencies, leads during operation to throughput-dependent mechanical and thermal loads on the hot-melt adhesives to be delivered. Furthermore, piston pumps have a highly pulsating delivery characteristic, which makes them unsuitable for continuous application methods. The use of conventional buffer tanks is restricted to fluids of comparatively low viscosity, since otherwise complete emptying under the effect of gravity is not ensured. Even sophisticated agitating elements or wall strippers cannot ensure in the case of tacky and viscous substances that residual material can be removed during emptying. Moreover, in continuous operation, back-mixing with the material already present in the buffer tank always occurs, which has the consequence that residence times vary widely and consequently thermally sensitive hot-melt adhesives are exposed to uncontrolled aging processes.
One particular special design of the buffer tank in connection with a gear pump is that of the gear-in-die (GID) principle. A slot die is in this case preceded by a gear pump which is constructed in the full coating width and draws the fluid to be coated from a similarly wide storage tank. The latter is of a rectangular construction, only partially filled and, if appropriate, subjected to compressed air. This gives rise to considerable disadvantages, in that, when highly viscous fluids are used, the fluid exchange in continuous operation and the emptying of residual material is possible only very incompletely on account of adhesion to the walls. At the same time, thermal-oxidative aging or even decomposition of sensitive fluids can easily occur over the surface of the fluid exposed to the air.
Pulsation dampers are customary instruments for equalizing pressure fluctuations in pipeline systems. However, in the case of incompressible fluids only very small equalizing volumes are required for this purpose, so that the available devices are not suitable for the equalization of relatively great differences in volume. They also have the disadvantage in principle that the material last received is the last discharged again, which can also have the consequence here of long residence times, and consequently unfavorable aging behaviour of heat-sensitive hot-melt adhesives.
The same disadvantage is encountered in the case of so-called buffer presses. Here, a movable, cylindrical tank is hydraulically pressed against a fixed plate which is fitted exactly into the cylinder and in which the inflow and outflow openings are located. Here, too, the fluid enters and leaves on the same side of the tank. As a result of the cylindrical construction, with a small length/diameter ratio and with an axially arranged inflow and outflows this type of apparatus has wide ranges with only very inadequate fluid exchange, which lead to the partial averaging of thermally sensitive or perishable fluids. With the same volume flows in the outflow and inflow, finally no exchange at all of the tank contents takes place. The mobility of the entire tank also makes sophisticated statics of the apparatus absolutely essential, in particular if the high discharge pressures of up to 30 bar are taken into account. Since a hydraulic transmission of the slender hydraulic cylinders to the large tank cross section is involved here in principle, such high pipeline pressures can only be achieved in the outflow with enormous hydraulic installations.
There are pressure accumulators for the equilibration of a fluid stream which are capable in continuous operation of equalizing both pressure and throughput fluctuations in a closed pipeline system and at the same time building up constant high processing pressures. The operating mode of the storage tank is based on the idea that the tank provides a certain volume of product and a certain free volume. This is available for the case in which the fluid-delivering or fluid-receiving side of the process fails.
The discharge of the fluid from the storage tank with a delivery element is assisted and kept constant by pressure build-up, for example by means of a pneumatically driven movable ram. A pressure peak caused by the fluid-delivering side of the process, caused by the volume deviating from the setpoint volume flow, is counterbalanced by the ram plate which is connected to the pneumatics releasing the difference in volume as tank volume. The admission pressure of the delivery element connected directly to the storage tank changes only to a negligible extent as a result, depending on the inertia of the system. The inertia of the system is dependent for example on the frictional moment of the ram seals and the volume of the pneumatic rams. Here it is advisable to use pneumatic rams which are of as large a volume as possible or equipped with a pressure-relief valve. This prevents compression of the gas in the pneumatic rams from taking place. The compression would cause a reaction on the ram plate which brings about an increase or decrease in the pressure in the storage tank and consequently in the admission pressure of the delivery element.
The ram is sealed with respect to the storage tank by suitable seals against escape of the fluid via the ram plate. These apparatuses, mostly derived from drum melting technology, exist in various configurations, which essentially differ by the arrangement of the points for feeding in and discharging the product.
It is known for the fluid to be fed in and discharged in the bottom region. The problems which occur in the case of this solution are analogous to those of the buffer press.
The object of the invention is to overcome the shortcomings of the technologies derived from the prior art for the equilibration of a temperature-sensitive highly viscous fluid stream by an apparatus which is capable in continuous operation of equalizing pressure and throughput fluctuations in a closed pipeline system, of ensuring a narrow range of controllable residence times, of storing large amounts of fluid and of building up constant high processing pressures.
Furthermore, the system is intended to have the following further properties:                It is to be largely free of any dead space, to avoid the aging of, in particular, temperature-sensitive fluids.        It is to be of a simple structural design for cleaning purposes and show a cost saving and also a high level of reliability to increase the overall efficiency of the installation.        
Application of the method is not restricted to highly viscous temperature-sensitive fluids, that is to say its use for fluids of low viscosity is not ruled out but equally possible.
This object is achieved by an apparatus such as that presented in the main claim. The subclaims describe advantageous developments of the subject-matter of the invention and also methods for the conserving intermediate storage of, in particular, temperature-sensitive highly viscous fluids in closed pipeline systems.